Wow I really cant believe the lack of love the defense is getting here. Not only do we have one of the best defensive coaches in the history of the game, but we have easily one of the best d-lines in the game as well as two of the top linebackers in Mayo and Thomas. A young and talented secondary with good veterans like Springs and Bodden brought into the mix as well.
Well, I think the problem is that the defense was pretty terrible last year, and has been declining pretty steadily for the last several years. They also have lost their three leaders of the past 7-8 years (Bruschi, Vrabel and Harrison).
Yes, they have improved their overall talent and speed on defense, and on paper, I believe they should be good. However, they also run very complicated schemes, and are going to be asking for a lot of young guys who have only shown flashes in the past at most (Merriweather, Guyton, Crable, Woods, Butler, Wheatley, Wilhite), and a couple veterans who are new to the system, and aren't really coming off career years (Bodden, Springs, Burgess) to play a ton of snaps. Really, the only sure things are the D-line as a whole, Mayo, and MAYBE James Sanders (although what you can be sure of with him is his mediocrity).
So while the defense has huge upside, and could be dramatically better, it also has some huge questionmarks. And given the lack of success the Pats have had bringing new guys into the defense over the last 5 years or so, there is plenty of reason to wonder how good the defense will be.
Now with that said, they could be one of the worst defenses in the league, and still go 12-4 IMO, because the offense will likely be THAT good.